1. Field of the Invention
The present invention relates to a continuously shifting-type steering system used for driving left and right driving wheels and for a differential steering of the driving wheels in an unleveled-ground working vehicle, a snow vehicle or other type of vehicle. In particular, the present invention is directed to an improvement in a continuously shifting-type steering system including a pair of left and right hydrostatic continuously variable transmissions each of which comprises a hydraulic pump and a hydraulic motor connected to each other through a closed hydraulic pressure circuit and which is capable of being individually shifted. Pump shafts of the left and right hydraulic pumps are synchronously driven by an engine, and a pair of output shafts connected to the left and right driving wheels are individually driven from the motor shafts of the left and right hydraulic motors.
2. Description of the Related Art
Such a continuously shifting-type steering system is already known, for example, as disclosed in Japanese Patent Application Laid-open No. 7-132743.
In the hydrostatic continuously variable transmission in such continuously shifting-type steering system, the capacity or displacement of the hydraulic pump is variable for the purpose of shifting the continuously variable transmission, so that the forward speed ratio is controlled by regulating the capacity of the hydraulic pump from zero to a positive maximum value. The shifting range is however, not sufficient. When a small-sized hydrostatic continuously variable transmission is employed, it is difficult to transmit larger power in a lower-speed range, due to the flow path resistance to the working oil and for this reason, the lower-speed output performance must be lowered.